Abstract Neurons in the brain can rapidly alter their transcriptional profiles in response to sensory inputs and intrinsic signals. Such experience-dependent changes in expression of genes that build and regulate synapses play a critical role in circuit development and information processing. We hypothesize that HDAC4, a member of the class II histone deacetylase family that shuttles between the nucleus and cytoplasm, controls a transcriptional program essential for synaptic plasticity and spatial memory. This hypothesis is based on the following preliminary results: i) the nuclear import of neuronal HDAC4 and its ability to interact with neuronal chromatin and transcription factors is negatively regulated by NMDA receptors; ii) in the nucleus, HDAC4 represses a restricted group of genes highly enriched in those known to be involved in synaptic function; iii) accumulation of HDAC4 in the nucleus affects both the architecture and strength of excitatory synapses; iv) a frame-shift mutation in the HDAC4 gene has been linked to a rare form of mental retardation in humans; and v) accordingly, mice carrying a truncated form of HDAC4 which mimics the mutant human allele exhibit deficits in neurotransmission and spatial memory. We propose to elucidate the role of HDAC4 in the brain using two unique animal models established in the laboratory. We have developed a new chemical-genetic system that enables drug-inducible control of glutamate release in live and behaving mice. We will take advantage of this system to define the role of synaptic inputs in regulating the localization and transcriptional activity of HDAC4 in vivo. In complementary studies, we will test the hypothesis that memory loss observed of HDAC4-deficient mutants is due to deficits in synaptic plasticity. To attain this goal, we will interrogae synapses of these mice using optical imaging and electrophysiology. We anticipate that these studies will provide important insight to molecular mechanisms of transcriptional control in neurons, and may eventually facilitate the design of new treatments of neurological diseases. !